Nitrogen trichloride avoidance

The pH value is usually maintained above 9 to avoid formation of nitrogen trichloride. At lower pH values, aqueous solutions react with chlorine to form cyanogen chloride (52). 

For the chlor-alkali industry, an emergency preparedness and response plan is mandatory for potential uncontrolled chlorine and other releases. Carbon tetrachloride is sometimes used to scrub nitrogen trichloride (formed in the process) and to maintain its levels below 4% to avoid fire and explosion. Substitutes for carbon tetrachloride may have to be used, as the use of carbon tetrachloride may be banned in the near future due to its carcinogenicity. 

The solvent-free material, isolated at — 70°C, disproportionates violently (sometimes explosively) at — 50°C to ammonium chloride and nitrogen trichloride [1]. Ethereal solutions of chloramine are readily handled [2], In the preparation of chloramine by reaction of sodium hypochlorite with ammonia, care is necessary to avoid excess chlorine in the preparation of the hypochlorite from sodium hydroxide, because nitrogen trichloride may be formed in the subsequent reaction with ammonia [3]. 

The mix of species produced depends on the ratio of chlorine to ammonia and the pH of the water. In the pH range of 7-8 with a chlorine-to-ammonia ratio (by weight) of 3 to 1, monochloramine is the principal product. At higher chlorine-to-ammonia ratios or at lower pH values (5-7), some dichloramine will be formed. If the pH drops below 5, some nitrogen trichloride (often erroneously called trichloramine ) may be formed. Nitrogen trichloride formation should be avoided because it imparts undesirable taste and odor to the water. To compare the disinfection efficiencies of secondary chlorination with secondary chloramination, a design engineer should also consider the effect of each process on corrosion control (see Appendix A). 

Reaction (75) is the first of a series of three that successively form dichloramine and nitrogen trichloride. Control of the pH allows the reaction to stop at the monochloramine stage. Section 9.1.11.2C on avoidance of NCI3 discusses this in mote detail. It also mentions that at high pH, where the hypochlorite ion is dominant, the end product can be nitrogen gas ... 

These data show that when the brine remains alkaline the concentration of HOQ is not too high- At pH 8.5, the major product is chloramine, which is quite volatile and can be stripped fixim the brine. At pH 4.5, the major product is nitrogen trichloride. This explains the formation of NCI3 in the cells, but acidic conditions are to be avoided when deliberately chltninating the brine. 

This procedure uses sodium hydroxide and pH has to be maintained at above 9, preferably pH 9 to pH 11, to avoid formation of toxic nitrogen trichloride (Kirk-Othmer, 1993). Hypochlorite neutralization is shown here (McGill and Comba, 1990) ... 

Cyanide spills can also be neutralized with peroxide by the Degussa method, in which the pH is maintained above 9 (between 9.0 and 10.5) to avoid formation of toxic NCI3, nitrogen trichloride. The Degussa method is shown below ... 

The chlorine gas can be cooled directly in packed towers. Water is sprayed into the top and flows countercurrent to the chlorine. This treatment thoroughly washes the chlorine however, dechlorination of the wastewater consumes a large amount of energy. The cooling water should be free of traces of ammonium salts to avoid the formation of nitrogen trichloride. 

When large amounts of chlorine gas are required or when the chlorination process needs pressurized gas, liquid chlorine must be vaporized and superheated to avoid liquefaction [243], [244]. It is advisable to operate the vaporizer at a sufficiently high temperature to accelerate the decomposition of nitrogen trichloride. As a source of heat, steam with a maximum allowed temperature of 120 C is used when the vaporizing system is constructed of mild steel. Water above 60 C is also suitable, as shown in Figure 84. Direct electrical heating is not appropriate because there is always a risk of overheating the steel. 

Every effort must be taken to avoid the reverse suction of water or organic materials into the vaporizer. The recommended water and nitrogen trichloride content of introduced liquid chlorine must not be exceeded. Vaporizers operating at low temperature or with a constant liquid level need to be purged to avoid dangerous concentration of nitrogen trichloride [246], [247]. 

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